In a telephone plant the data required for sizing plant requirements and for monitoring the grade of service in its various parts consist mainly of three main types: Usage, congestion, and number of calls. Three different facilities are usually used to collect that data. The facility for usage measurement consists of traffic usage recorders of which various electromechanical and electronic types are available on the market. They can be broadly classified into two main categories, analog and digital. In the analog, the sum of the current drain of busy circuits is continuously recorded. In the digital, the circuits are scanned at regular intervals and the number of busy circuits (called busies hereafter) is scored on digital counters which sum their number during a period of one hour. The measured load is expressed in convenient units of telephone traffic such as CCS (hundred-call-seconds) or Erlangs.
Various measurements also are used for the monitoring of call congestion. These are: peg count, overflow, all trunks busy (ATB) and last trunk busy (LTB). Overflow measurement consists of scoring the number of call attempts which arrive when all the circuits in the group under consideration are occupied. The ratio of such unsuccessful call attempts to all call attempts (which are measured by a peg count counter) is called the grade of service or percent of no circuit (%NC), and is used as a measure of congestion. Theoretically, the grade of service is defined as the limit (as time increases without bound) of the ratio of the overflow and peg count counter readings. In ATB measurement one counter per group of trunks measures the duration of the all trunks busy (ATB) condition. Similarly, in LTB measurement a counter measures the duration of the last trunk busy condition. The grade of service can then be obtained from that data by mathematical formula. The accuracy of congestion measurement by these methods is very poor due to the practical limitation of the observation period. Moreover, they are only applicable to full availability trunk groups.
The facility for measuring the number of calls consists of call counters which score the number of call attempts. Usage can be estimated as the product of the number of calls and their average duration, a quantity that is obtained by another type of measurement.
The above mentioned methods were used traditionally since the Twenties. They served their purpose until the introduction of automatic alternate routing in the Long Distance Dialing Network in which final routes are offered "peaked" traffic identified with variance greater than the mean, as opposed to random traffic identified with variance equal to the mean. Peaked traffic requires more service channels than random traffic for the same load and grade of service. The ratio of the variance to the mean of the offered traffic is called the peakedness factor (PF).
At present, estimates of the offered load and its PF are made from usage and %NC (no circuit) data. Due to the limitation of the observation period and the presence of retrials (reordered calls), especially at times of congestion, the reliability of the estimate by this method is very poor, besides being costly. Moreover, usage measurement as described above is subject to another inherent inaccuracy, in that the load is measured during fixed hourly periods rather than during moving or sliding hourly periods. (Theoretically, the busy hour load is defined as the maximum load during a period of sixty consecutive minutes or part thereof.) Add to this the difficulty in practice of updating (augmenting) the measuring leads with changes in trunk group size and the processing effort of a voluminous amount of measured data.
This invention by means of a new conceptual approach cures the infirmities of present-day methods and has the following salient advantages:
(1) Provides the mean and variance of the number of calls occurring in a period of 60 consecutive minutes or part thereof during periods of highest traffic intensity.
(2) Provides the mean and variance of the load in cooperating groups of trunks forming a cluster which may have non-coincident busy hours during the significant cluster busy hour.
(3) Provides estimate of the mean and variance of the offered load and its blocking probability with greater accuracy. While the method of calculating the offered load and its variance from the mean and variance of carried load on a selected group of trunks is known, no device for practical application of the theory is believed to exist.
(4) Provides economy in the cost of traffic measurement by limiting the measurement to a limited number of trunks which are arranged to be used at last-choice trunks. The measuring leads will therefore be permanently connected to the device and need not be changed with trunk group growth. This overcomes the present difficulty of rearrangement of the measuring leads.
(5) Provides the facility for monitoring, not only overload conditions to guard against service deterioration, but also for monitoring plant overprovision, a feature which is required in a monopolized public service. The load carried by the last-choice trunks provides tangible evidence of plant overprovision or underprovision. Present systems do not effectively provide the facility to measure overprovision.